Emission of sulfurous flue gasses from industrial activities utilizing coal fired boilers and other processes are a worldwide ecological problem. SO2 emissions from the flue gas of coal-fired power plants and the like have made the United States a major contributor in this regard.
A process, wherein the SO2 is selectively reacted with a finely divided limestone slurry, is well known as wet flue gas desulfurization or WFGD. In this process, limestone is pulverized in a ball mill to provide the surface area needed to achieve the desired reaction kinetics. This limestone is mixed with water to produce a pumpable slurry and then utilized in a counter current reactor, or “absorber”, to come into intimate contact with the flue gas being treated. Ball mill operations are power intensive, so the degree of particle size reduction must be optimized by balancing the milling costs against the efficiency improvement of the scrubbing process.
In many prior art embodiments of the WFGD process, dibasic acids (DBA) are added in the absorber stage as a catalyst and pH buffer to chemically enhance the reaction kinetics and thus the efficiency of the desulfurization process. DBA, which is both a DuPont product and a DuPont tradename, is commonly found as a byproduct stream made up of Glutaric, Succinic, Adipic and Nitric acids, coming from the production of adipic acid as a precursor to Nylon™. Monsanto produces a similar byproduct stream, having the trade name of AGS (also called DBA, colloquially, by users in the market place). Each of these byproduct stream components, except for the nitric acid, has very limited water solubility, so that the typical mixture used by the utilities must be shipped and stored as a 50% water solution at about 160 degrees F. in order to stay in solution.
In the absorber, SO2 in the flue gas reacts with the limestone (Ca CO3) slurry so as to remove the SO2 by converting it to Ca SO4 (gypsum). CO2 is produced as a by-product of this reaction, and exits the absorber with the rest of the now de-sulfurized flue gasses. As is well known to those skilled in the art, DBA acts to catalyze the reaction of SO2 in the flue gas with the calcium carbonate in the aqueous phase of the absorber slurry.
Calcium carbonate has a very low solubility in water, which is problematic since the reaction between it and the SO2 must take place in the aqueous phase of the absorber. This problem is overcome when the calcium carbonate comes into contact with the DBA, because it reacts with the DBA to produce calcium salts with enhanced water solubility, specifically calcium adipate, calcium succinate and calcium glutarate. As stated above, since the SO2 can only react with the calcium in the aqueous phase of the absorber, the higher the concentration of water soluble calcium salts in the absorber slurry, the greater the number of calcium ions that come into contact with the SO2, the more efficiently the SO2 is removed and the resulting gypsum is formed.
Since the DBA acts only as a catalyst in this process, it is unchanged and reverts to its acid form as the reaction is completed and the gypsum is formed, remaining in the water solution, available to react with other calcium carbonate molecules. As the residual slurry is de-watered, the water containing the now reformed DBA and/or any newly formed calcium salts is then returned to the absorber for reuse. The de-watered gypsum may then be used for making wallboard or for other purposes, or simply land filled.
It has been shown that without the intermediate formation of salts resulting from the reaction of calcium carbonate with the DBA catalyst, the transfer of the conversion of SO2 and calcium carbonate to Ca SO4 (gypsum) is difficult, time consuming and inefficient. Even so, there are significant cost and maintenance issues with DBA, because:
a.) Even though the DBA reaction with calcium carbonate produces calcium salts whose solubility is greater than either calcium carbonate or DBA alone, these calcium salts still have somewhat limited water solubility; and
b.) Under the conditions present in the absorber system, the DBA experiences rapid degradation and therefore significant volumes of new DBA catalyst solution must continuously be added to maintain sufficient concentrations in the absorber liquid to promote the removal of SO2; and
c.) The corrosive nature of DBA can promote erosion and/or corrosion in the absorber over the long term so as to become a maintenance issue. Because of its corrosive nature, DBA is considered to be a hazardous material, with safe transportation, handling and environmental issues, all of which add to the total process costs.
The Environmental Protection Agency has imposed ever-tighter regulations on sulphurous emissions and may be expected to continue to do so in the future, so that process efficiency and overall cost will become increasingly more important. In any case, process cost will be a continuing concern.
It would seem logical to add DBA to the limestone at the grinding stage, so as to reduce ball mill frictional losses and begin the conversion of calcium carbonate to calcium salts earlier in the process. This is not feasible however, since DBA quickly breaks down in the ball mill and becomes ineffective as a catalyst.
A first object of the present invention is, therefore to reduce WFGD process and maintenance costs. A second object is to eliminate or mitigate safety and environmental issues involved in the process. Yet a third object is to enhance the efficiency of the desulfurization process.